1. Field of the Invention
The present invention relates to a magnetic fluid seal apparatus for establishing a hermetic seal between relatively movable housing and shaft. The present invention is particularly useful in hermetically sealing a shaft extending between a vacuum and an atmospheric environment and arranged relatively for linear reciprocating movement.
2. Description of the Prior Art
Magnetic fluid seal apparatus have been used to provide hermetic seals around movable shafts. A magnetic fluid seal apparatus includes a magnetic fluid that contains ferromagnetic particles dispersed in a carrier liquid. The magnetic fluid is magnetically retained in an annular magnetic gap formed between the shaft and the housing. Where the magnetic fluid seal apparatus is used to seal a shaft which undergoes a reciprocating linear motion, a small quantity of the magnetic fluid tents to be dragged and undesirably displaced along the surface of the shaft or the housing, thereby resulting in the contamination of the vacuum environment, such as a vacuum chamber. Furthermore loss of the magnetic fluid due to displacement may eventually result in the failure of the sealing capability. The manner in which such undesirable displacement of the magnetic fluid takes place is schematically illustrated in FIG. 1 of Japanese Patent Kokai Publication No. 58-68554/1983.
In order to offset these disadvantages, the above-identified patent publication contemplates to reduce the surface wettability of the shaft and, to this end, proposes the use of a coating 6 of nonpolar polymer applied over the shaft 12 in combination with the use of a magnetic fluid 11 having a high surface tension, as shown in the first embodiment thereof illustrated in FIG. 2 of that publication. This solution, however, is not satisfactory because the reduced surface wettability causes the magnetic fluid to be repelled from the surface thereby permitting the release of the magnetic fluid. Therefore, that publication further provides a plurality of scraper blades 7 and 17 of elastomeric material, as shown in FIGS. 3 and 4 of the publication. However, the provision of the scraper blades is disadvantageous because the shaft is subjected to an increased frictional resistance so that the linear motion of the shaft is obstructed.